The three common standards against which arguments are evaluated are: TRUTH, VALIDITY AND SOUNDNESS. An argument is said to be valid if the premise of the argument entails its conclusion, that is, if the truth of the premise makes the conclusion to be true also. An argument is said to be sound, if it expresses a valid thought and its premises are true.
I think it was because we really don't know what exactly happened in the past. Also, I'm pretty sure people did not keep track of colored people back then.
